Digital step attenuation (DSA) can be used in the transmit (TX) and receive (RX) signal chains of a wireless (RF) communication transceiver or system to dynamically attenuate/adjust RF signal power to be within the input range specification for TX power amplification (PA), or RX conversion to digital (ADC). Gain control functionality can be used to dynamically select DSA attenuation steps, each step to provide a specified level of attenuation (in dB).
An important performance requirement for a DSA is to dynamically control attenuation without distorting the attenuated TX or RX signal input to the ADC. Particularly for wireless infrastructure applications, specifications on the accuracies of the DSA attenuation steps can be stringent.
For example, 3G/4G systems employ QAM (I/Q) constellations for encoding bits. A phase/gain error in DSA attenuation can cause a change in the constellation points, leading to errors in symbol decoding. Specifically, a gain error will lead to expansion/shrinkage of the constellation points, and a phase error will lead to rotation of the constellation points.
Within a target frequency band (band of interest), phase and gain errors can vary with frequency. One approach to compensating for phase and gain errors is to include in the post-conversion digital signal chain an N-tap complex filter can be used to correct for frequency dependent mismatches. To estimate the filter taps, a tone is swept through the target frequency band, and the estimated taps are programmed into an LUT indexed by DSA attenuation word generated by the AGC.
While this Background is directed to digital step attenuation in the context of RF communications (TX/RX), the Disclosure has general applicability to providing digital filtering for frequency-dependent errors in phase and/or gain of an analog signal.